A study has been made of the influence of the conditions of modification of an ultrastable Y zeolite produced by a method including treatment of the zeolite with a solution of ammonium hexafluorosilicate on the physicochemical and catalytic properties of the zeolite. IR spectroscopy and temperature-programmed desorption of ammonia have been used to study the effect of acid and alkali treatment of an ultrastable Y zeolite on its crystal structure and acidic properties (overall acidity, the strength distribution of acid centres). The catalytic activity of modified zeolites was assessed from the yield of fuel fractions (gasoline and diesel) during the hydrocracking of vacuum gas oh. It has been shown that the method of zeolite modification makes it possible to control its acidic properties as required. The yield of fuel fractions is determined both by the nature of the acid centres and the ratio of medium-strength and strong centres and by the process conditions. Depending on the demand for preferential production of a particular fuel fraction, the process must be carried out under optimum conditions and on a catalyst with which the ratio between medium-strength and strong acid centres is optimum.